Problem: Suppose the area of a circle is ${25\pi}$. What is its diameter?
${K = 25\pi}$ ${d = 10}$ First, find the radius: $K = \pi r^2$ , so $r = \sqrt{K / \pi} = \sqrt{{25\pi} / \pi} = {5}$ Now find the diameter: $d = 2r = 2\cdot {5} = {10}$.